Shroud structure for turbine rotor blades and the like

ABSTRACT

An improved honeycomb shroud for rotor blades for turbines and similar devices, wherein the rub-tolerance of the shroud is increased by making the honeycomb less rigid and more resilient and yieldable at the potential rubbing surface, with added strength at the region of attachment to the support member.

Elite Moslrowitz 1 Feb. 18, 1975 SHROUD STRUCTURE FOR TURBINE 3,068,016 12/1962 Dega 415/174 ROTOR BLADES AND THE LIKE 3,126,149 3/1964 Bowers et a1. 415/174 3,583,824 6/1971 Smuland et al. 415/174 Inventor: Seymour Mflslwwitz, Fort 3,694,882 10/1972 Desmond 415/174 Assignee: curtiss wright Corporation, 3,728,039 4/1973 Plemmons et al. 415/174 Wood-Ridge, NJ. Primary Examiner-C. J. Husar [22] Flled' 1973 Assistant Examiner-Louis J. Casaregola [21] Appl. No.1 427,652 Attorney, Agent, or Firm-Raymond P. Wallace [52] US. Cl. 415/174, 415/135 [57] ABSTRACT lqi j g f -g 1 An improved honeycomb shroud for rotor blades for 3 turbines and similar devices, wherein the rubtolerance of the shroud is increased by making the honeycomb less rigid and more resiilient and yieldable [56] References Cited at the potentlal rubbing surface, wlth added strength U/NITED STATES PATENTS at the region of attachment to the support member. 2,477,852 8 1949 Bacon 415/174 3,053,694 9/1962 Daum et a1. -6 Claims, 6 Drawing Figures TRAVE L FLUID our PATENTEB m1 1 81975 ,SHEU 10F 2.

FIG. 2

PATENIEBFEBI ems SHEET 2 OF 2 SHROUD STRUCTURE FOR TURBINE ROTOR BLADES AND THE LIKE BACKGROUND OF THE INVENTION This invention relates to gas turbine engines, steam turbines, and rotary pumps, and more particularly to an annular shroud structure surrounding the tips of the rotor blades of such devices to prevent fluid leakage.

In devices having a fluid coacting with rotor blades, such as gas turbine engines, steam turbines, and rotary pumps, the efficiency can be materially increased by minimizing the leakage of fluid between the tips of the rotor blades and the surrounding shroud structure. The clearance between the blade tips and the shroud is made large enough in the cold state to allow for thermal expansion of the parts, machining tolerances, concentricities, radial looseness in the bearings, etc., with some additional allowance for unknown idiosyncracies which may develop in any given machine.

In devices with large rotor diameters, that is, those wherein the radial height of the blades is of the order of three inches or more, it is usual to provide a clearance of about 1.5% of the blade height. Smaller machines must use a greater percentage allowance for clearance, with an attendant decrease in efficiency, since the absolute clearance is substantially affected by the unavoidable fabricational factors which do not diminish in proportion to the reduction in size of theapparatus.

Thus, in both large and small turbines some type of abradable or other yieldable shroud around the blade tips is desirable so that the lowest possible cold clearance can be provided, and so that if rubbing of the blade tips against the shroud occurs during steady-state operation the rubbing damage will be minimal. A common expedient in the prior art is to form the shroud of honeycomb material with the edges of the honeycomb cells juxtaposed to the blade tips, as shown in US. Pat. No. 3,426,665. This minimizes the actual rubbing surface of the shroud in case of contact with the blades.

However, such honeycombs of the prior art are formed with hexagonal cells, each of which constitutes a relatively rigid box, which is backed up by the adjacent boxes, and these boxes are not very resilient and do not easily move out of the path of the rotating blade tip. Cases have been observed wherein the blades are severely scored and even notched from such rubbing.

The present invention overcomes such limitations of the prior art by providing a flexible honeycomb structure.

SUMMARY The present invention provides a honeycomb shroud structure for gas turbine engines, steam turbines, rotary pumps, and the like, wherein three sides of each hexagonal cell have a greater radial height and yieldability than the remaining three sides which provide structural rigidity at the base. Thus, the surface of the shroud juxtaposed to the blade tips where rubbing may occur comprises rows of flexible convoluted ribbon with their edges generally transverse to the line of contact with the blade tips. These ribbons are unsupported in the travel direction so that in the event of light rubbing the ribbons merely flex out of the way. If the rubbing should be severe the ribbons may be permanently deformed in the generally circumferential direction, but

only to the extent of their interference with blade travel, so that the blades will not be damaged and in any subsequent operation the clearance at the rubbing point will be substantially zero.

It is therefore an object of this invention to provide a novel and improved shroud structure for apparatus having bladed rotors.

Another object is to provide such a shroud structure wherein a minimum clearance between the blade tips and the shroud may be provided in the cold state without danger of seizing during steadystatc operation.

A further object is to provide SLllCh a shroud structure having honeycomb cells which are flexible at the surface apposed to the blade tips.

Other objects and advantages will become apparent on reading the following specification in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a general view of a gas turbine engine, partially cut away to show the location of the turbine rotor and the novel shroud structure;

FIG. 2 is a fragmentary view on an enlarged scale of a blade tip and the shroud structure;

FIG. 3 is a much enlarged view taken generally on line 3--3 of FIG. 2;

FIG. 4 is a fragmentary cross-section taken on line 44 of FIG. 3;

FIG. 5 is a cross-section similar to FIG. 4 of a modified embodiment; and

FIG. 6 is a schematic view showing the relation of the blade tips and their path of travel with regard to the shroud. 1

DESCRIPTION OF A PREFERRED EMBODIMENT Although the invention will be described principally in combination with a gas turbine engine, it is to be understood that it is equally applicable to steam turbines, rotary pumps, and other fluid devices having rotors with radially extending blades.

In FIG. 1 there is shown a turbine engine 11 of the type used in aircraft. The basic elements of the engine are conventional, comprising an outer housing 12 and at least one axial shaft 13 having a turbine rotor 14 mounted thereon at the aft end in driving relation thereto, the rotor having a plurality of radially extending blades 16 which rotate between guide vanes 17 disposed upstream and downstream therefrom. Within the housing is provided a combustion chamber 18 which receives air from the compressor (not shown) which is driven by shaft 13. Fuel from a fuel supply (not shown) is delivered to the combustion chamber by nozzles 19 where it is mixed with air and ignited, the resulting combustion gases driving the turbine rotor and subsequently being exhausted through the exhaust nozzle 21. The combustion chamber 18 is extended in the downstream direction to the region of the first guide vanes 17 and defines the radial dimension of the gas path, the combustion gases traveling between the guide vanes 17 and the rotor blades 16 in the direction shown by the arrow to drive the turbine rotor. A shroud structure 22 circumferentially surrounds the rotor blades 16, the shroud being supported and positioned by the outer shell or other convenient structure. Other elements, accessories, and appurtenances of such an engine are omitted from the drawing as not being essential to an understanding of the invention.

FIG. .2 shows an enlarged cross-sectional view of the radially outer end of a guide vane 17 and the radially outer end of a rotor blade 16, the section being taken in a plane parallel to the engine axis and looking in the circumferential direction of blade rotation. The general direction of the gas path across the guide vane and the rotor blade is shown by the arrow. The shroud structure 22 comprises a structural back-up ring 23 formed of material having high strength at elevated temperatures, supported in any convenient manner, such as by the rings 24 interlocking with rings 26 extending inwardly from the shell 12. The shroud proper 27 comprises a honeycomb (described below) mounted on the backup ring and having the edges of its cells presented to the tips 28 of the rotor blades. Such annular shrouding structures are usually segmented in the circumferential direction with overlapping ends to avoid the hoop effect and to allow thermal expansion of the segments.

The gap 29 between the blade tips 28 and the shroud 27 is a potential source of leakage of the fluid passing through the machine. Such leakage may occur because fluid traveling in the general flow direction from the stator vane to the rotor blade zone passes radially outside the rotor blades across the tips. Leakage may also occur because of pressure differentials within the blade passages. The concave sides of the rotor blades are high pressure sides where the fluid impinges, the convex sides being known as the suction sides, there being a lower pressure along the convex sides than along the concave sides. Therefore, fluid close to the blade tip may be drawn across the tip from the concave to the convex side and not coact in the desired manner with the blade. For this reason it is desirable that gap 29 should be as small as possible in order to avoid loss of fluid by either mode.

FIG. 3 shows a much enlarged view of a small portion of the shroud member 27, displaying the surface which has the edges of the honeycomb cells juxtaposed to the blade tips. The proportions in this view are much exaggerated for clarity of illustration. The cells in such a honeycomb are usually of the order of about onefourth inch to 3/16 inch diameter across the flats, but may be somewhat more or less. The material of the cell walls may be from about 0.004 inch to about 0.012 inch thick, and is frequently about 0.008 inch. Suitable materials for the cell walls are the nickel-base heatresistant alloys, or other similar alloys having sufflcient temperature resistance but not great hardness. The alloys known as Hastelloy X, Inconel X, and similar compositions are suitable.

Such a honeycomb may be formed by various procedures. In one method, strips of the material are convoluted over their length into semihexagonal form and positioned on edge on a segment of back-up ring 23 against and paralleling other similar strips with the convolutions opposed, then brazed or resistance welded in place. Alternately, the strips may be brazed or welded together at their abutting convolutions to form a network, which is then placed on the supporting material and affixed thereto by brazing or welding.

Either procedure, if carried out with strips of equal width, or height as they are placed on the backing material, produces a plurality of hexagonal boxes supporting each other, with double walls on two sides of each cell, and by their geometrical configuration being very strong and rigid. Such a honeycomb in the prior art has resulted in severe scoring and notching of the blade tips when rubbing has occurred.

In this invention strips of two different widths ar used to form the honeycomb. with each wider strip opposing a narrower one, so that the cells are complete hexagons only in their lower portions adjacent to the back-up base 23, but at the rubbing surface there are presented a plurality of unsupported edges of parallel convoluted strips.

This construction is shown in FIGS. 3 and 4. Wide convoluted strips 31 are positioned on edge, paralleling and abutting alternate narrow strips 32 with opposed convolutions. When strips 31 and 32 are brazed or welded in place the hexagonal cells are closed only in the bottom portion adjacent to the base 23. The wider strips 31 stand free to a greater height, so that at the rubbing surface these strips are unsupported by a hexagonal configuration. The, proportions of width of the two sizes of strips may vary, depending on the depth of honeycomb desired, the thickness of the-strip material, the size of the apparatus in which the shroud is to be used, and the degree of flexibility desired at the rubbing surface. As a general proposition the narrow strips 32 should be from about one-half to about three-quarters of the width of the wide strips 31. However, there may be some variation from these proportions in accordance with the factors set forth above.

It is possible for a slight amount of fluid to leak past the blade tips 28, by either of the modes described, when the shroud shown in FIGS. 3 and 4 is used. Also, the open pockets of the honeycomb cells may cause a slight amount of turbulence in the region of the blade tips. For many applications the amount of leakage and turbulence arising from these factors is unimportant, but may be more critical in other cases, such as in small turbines wherein the proportion of gap to blade height is greater, and where the size of the honeycomb cells cannot be reduced in proportion to blade dimensions, owing to fabricating difficulty. In such cases it is desirable to provide still further sealing efficiency.

FIG. 5 shows a modified embodiment of the invention which increases the sealing effectiveness. The hexagonal portions of the cells and the zigzag channels between the freestanding portions of strips 31 are filled with a yieldable material 33, which may be allowed to stand slightly higher than strips 31, or may be surfaced level with the strips, or may be slightly below them. The yieldable filler material 33 may be a friable and abradable material such as a porous, spongy ceramic, or felted metal fibers, or it may be a compressible material such as metal sponge, which is less likely to release tiny fragments when rubbed than the other materials.

A very satisfactory yieldable filler material 33 is a nickel-chromium-iron heat-resistant alloy in the form of a soft, compressible metal sponge which is not only strong and tough but resistant to corrosion and to the temperatures encountered in turbine operation. Such compressible metal sponge material is sold by the Wall Colmonoy Corporation under the trademark Nicroseal. It is very light in weight, having a specific gravity of approximately 2, compresses readily by any contact with the rotor blade tips, and has little tendency to release particles. It can be compressed to approximately 40 percent of its original thickness. Such a yieldable metal sponge filler 33 is bonded to the base 23 and to the strips 31 and 32, and will withstand the operating temperatures and erosive action of turbine fluids. If rubbing should occur during operation, the surface of the material will compress at the point of contact and conform to the shape of the rubbing element. Since the filler material between the strips 31 is compressible, it does not provide sufficient resistance to materially stiffen strips 31, but yields to allow them to flex or deform to allow passage of the blade tip. Thus the strips operate in the same manner as if no filler were present, but the spaces between are substantially filled so that no turbulence occurs.

It is important that the strips 31 should be properly oriented with respect to the direction of travel of blade tips 28. If the strips 31 were disposed on the segments of the backing ring 23 in such a manner that the length of the strips extended in the direction of blade tip travel, when rubbing occurred the same portion of the blade tip would traverse a considerable length of the strip. Further, since the strip edges have little flexibility in their longitudinal direction, such an edge would then approximate a wavy knife edge and might cause considerable damage to the blade.

A suitable orientation is schematically shown in FIG. 6, wherein the high strips 31 are indicated by solid zigzag lines and the lower strips 32 by zigzag dashed lines. The blade orientation at the tip ends 28 is superimposed in dashed lines, the direction of blade travel being indicated by an arrow bearing a legend.

Since turbine blades have a twist between root and base, the chords of the tips are neither parallel to the axis of rotation nor transverse thereto, but at some angle between, depending on the particular device. Therefore, although the shroud structure is concentric with the turbine rotor, it is preferable to dispose strips 31 with their convoluted longitude approximately parallel to the chordal centerline of the blade tips, as shown in FIG. 6. This orientation allows the blade to flex the strip in the direction of travel. If the rubbing contact is not entirely along the chord of the blade tip but only in a portion thereof and is not too deep, only a portion of the freestanding strip will be rubbed, and the convolutions will straighten out resiliently as in rubbing an edge of a partially opened paper fan having accordion pleats. The strip 31 then returns to itsoriginal configuration until the nextcontact. If the amount of interference is deep enough the edge ofthe strip will be bent over and take a permanent configuration, but with at least some resilient return, so that only grazing contact occurs at that point thereafter and the sealing surface is maintained at the proper radial distance from the rotor center for good sealing during steady-state conditions.

The strips 31 need not be oriented precisely with their longitude parallel to the chordal centerline of the blade tips, there being considerable latitude allowable without losing the advantages of the improved shroud structure. However, especially when using the shroud without filler material, the convoluted strips 31 should be suitably oriented with respect to the direction of fluid flow. Although the over-all general direction of fluid flow through the machine across the stator vanes, rotor blades, and exit vanes is axial, as indicated by the arrow in FIG. 2, the fluid does not enter the rotor blade passages in an axial direction. It is directed by the upstream stator vanes against the concave sides of the rotor blades 16 at an angle, as indicated in FIG. 6 by the arrow bearing the legend FLUID IN, travels between the rotor blades in the generally axial direction,

6 and leaves as shown by the arrow bearing the legend FLUID OUT. The FLUID IN arrow is intended to indicate only general angularity of the entrance direction but not any specific angle, which may vary from one machine to another.

Strips 31 should therefore not be oriented with their length in the direction of entering fluid, which would allow some fluid to pass through" the channels between strips. They should be oriented to present their side surfaces to entering fluid, either approximately normal thereto as shown in FIG. 6, or at some angle thereto, such as 30 or more. Also, they should not be disposed with their longitude sufficiently close to the axial direction to provide a clear sight in the axial direction between strips. As the fluid flow is turned in the generally axial direction by the rotor blades, such an arrangement would again allow fluid to flow directly through the channels. In the arrangement shown in FIG. 6 it will be seen that there is no clear channel through in the generally axial flow direction, at least two strips being shown as traversed by an axial straight line. It should also be borne in mind that the showing of FIG. 6 very much exaggerates the size of the hexagonal convolutions, and in actual equipment many such strips would be traversed in an axial straight line.

In the case where filler material 33 is used in the hexagonal cells and in the channels between strips, the foregoing considerations of the angular orientation of the strips are of less importance with respect to the direction of fluid flow, since even if the filler were slightly below-the level of the edges of the strips comparatively little fluid would get into any remaining space between strips.

What is claimed is:

1. In a fluid flow device having a rotor with generally radially extending blades coacting with fluid flowing between the blades, a shroud structure circumferentially surrounding the tips of the rotor blades in close proximity thereto to minimize leakage of fluid across the blade tips, the gap between the blade tips and the apposed surface of the shroud structure having in the cold state of the device a radial dimension which may diminish during operation sufficiently to cause rubbing contact between the blade tips and the shroud, the improvement comprising:

a. the shroud having a plurality of strips convoluted in a semi-hexagonal configuration in their longitudinal direction, each of the strips having one edge affixed to a rigid backing element and its opposite edge freestanding in the radial direction and apposed to the blade tips,

b. the strips being disposed with their convoluted length approximately parallel to the chordal centerline of the rotating blade tips,

c. the freestanding edges of the strips being resilient to rubbing contact with the blade tips in the generally circumferential direction of the shroud and generally transversely to the convoluted longitude of the strips.

2. The combination recited in claim 1, wherein a second plurality of semihexagonally convoluted strips are affixed to the backing element in alternating relationship with the first plurality of strips, the second strips having their convolutions opposed to the convolutions of the first strips to form hexagonal cells therewith, the second strips having less radial height than the first strips so that the hexagonal cells have less radial height than the freestanding edges of the first strips.

3. The combination recited in claim 2, wherein the over-all direction of fluid flow through the device is in a direction generally parallel to the axis of rotor rotation, and the first strips are disposed at such an angle to the axis that there is no clear line of sight in the axial direction between the freestanding edges of the first strips.

4. The combination recited in claim 3, wherein fluid enters the passages between rotor blades at an angle to the axis, and the freestanding strips are disposed with sponge. 

1. In a fluid flow device having a rotor with generally radially extending blades coacting with fluid flowing betweeN the blades, a shroud structure circumferentially surrounding the tips of the rotor blades in close proximity thereto to minimize leakage of fluid across the blade tips, the gap between the blade tips and the apposed surface of the shroud structure having in the cold state of the device a radial dimension which may diminish during operation sufficiently to cause rubbing contact between the blade tips and the shroud, the improvement comprising: a. the shroud having a plurality of strips convoluted in a semihexagonal configuration in their longitudinal direction, each of the strips having one edge affixed to a rigid backing element and its opposite edge freestanding in the radial direction and apposed to the blade tips, b. the strips being disposed with their convoluted length approximately parallel to the chordal centerline of the rotating blade tips, c. the freestanding edges of the strips being resilient to rubbing contact with the blade tips in the generally circumferential direction of the shroud and generally transversely to the convoluted longitude of the strips.
 2. The combination recited in claim 1, wherein a second plurality of semihexagonally convoluted strips are affixed to the backing element in alternating relationship with the first plurality of strips, the second strips having their convolutions opposed to the convolutions of the first strips to form hexagonal cells therewith, the second strips having less radial height than the first strips so that the hexagonal cells have less radial height than the freestanding edges of the first strips.
 3. The combination recited in claim 2, wherein the over-all direction of fluid flow through the device is in a direction generally parallel to the axis of rotor rotation, and the first strips are disposed at such an angle to the axis that there is no clear line of sight in the axial direction between the freestanding edges of the first strips.
 4. The combination recited in claim 3, wherein fluid enters the passages between rotor blades at an angle to the axis, and the freestanding strips are disposed with their longitude at an angle to the direction of fluid entering the blade passages.
 5. The combination recited in claim 2, wherein the hexagonal cells and the spaces between the freestanding edges of the first strips are filled with a yieldable filler material to approximately the height of the freestanding edges.
 6. The combination recited in claim 5, wherein the yieldable filler material is a compressible metallic sponge. 